Problem: On the first 3 tests of her chemistry class, Jessica got an average score of 77. What does she need on the next test to have an overall average of 82?
Explanation: Let her score on the next test be $x$ The sum of all of her scores is then $3 \cdot 77 + x$ The same sum must also be equal to $4 \cdot 82$ Solve: $x = 4 \cdot 82 - 3 \cdot 77 = 97$.